Sensitized paper plates for producing planographic printing forms

ABSTRACT

Inexpensive positive-imaging sensitized plates, easily convertible into good and durable planographic printing forms by imagewise exposure and development with an aqueous buffered phloroglucinol solution at a pH of 4 to 8, are provided by forming on a paper support a planographic coating consisting essentially of a finely divided inorganic filler and, as a hydrophilic organic binder, water-insolubilized polyvinyl alcohol having a degree of hydrolysis greater than 98 percent, and sensitizing this coating with a benzene diazonium salt of high coupling activity. The coating is formed from an aqueous dispersion containing a cross linking agent for insolubilizing the polyvinyl alcohol, and is dried and hardened to a Cobb-test water absorbing capacity of less than 6. The plates give extremely long-running offset printing forms when exposed through a transparent original, and even when the original is a typewritten translucent sheet.

United States Patent Hilhorst [451 Apr. 25, 1972 [54] SENSITIZED PAPER PLATES FOR PRODUCING PLANOGRAPHIC PRINTING FORMS [72] Inventor: Aloysius Henricus Jacobus Hilhorst, Hel- [30] Foreign Application Priority Data Jan. 12, 1968 Netherlands ..68,00539 [52] US. Cl ..96/75, 96/33, 101/462, 96/85 [51] Int. Cl ..G03c 1/52, G03f 7/02 [58] Field of Search ..96/33, 75, 85

[56] References Cited UNITED STATES PATENTS 3,128,181 4/1964 Doggett ..96/33 3,301,679 l/1967 Halperin et a1. ....96/75 3,330,662 7/1967 Jahoda et a1 ....96/75 3,409,434 1 H1968 Landberge et al. ..96/75 FOREIGN PATENTS OR APPLICATIONS 1,038,279 8/1966 GreatBritain 1,082,889 9/1967 Great Britain 1,064,128 4/1967 Great Britain OTHER PUBLICATIONS Tappi Test Method T441 08-69 Primary Examiner-Norman G. Torchin Assistant Examiner-John Winkelman Attorney-Albert C. Johnston, Robert E, Isner, Lewis H. Eslinger and Alvin Sinderbrand [57] ABSTRACT Inexpensive positive-imaging sensitized plates. easily convertible into good and durable planographic printing fonns by imagewise exposure and development with an aqueous buffered phloroglucinol solution at a pH of 4 to 8, are provided by forming on a paper support a planographic coating consisting essentially of a finely divided inorganic filler and, as a hydrophilic organic binder, water-insolubilized polyvinyl alcohol having a degree of hydrolysis greater than 98 percent, and sensitizing this coating with a benzene diazonium salt of high coupling activity. The coating is formed from an aqueous dispersion containing a cross linking agent for insolubilizing the polyvinyl alcohol, and is dried and hardened to a Cobbtest water absorbing capacity of less than 6. The plates give extremely long-running offset printing forms when exposed through a transparent original, and even when the original is a typewritten translucent sheet.

14 Claims, No Drawings SENSITIZED PAPER PLATES FOR PRODUCING PLANOGRAPI'IIC PRINTING FORMS This invention relates to planographic printing plates of the kind having on a paper or like support a light-sensitive planographic coating consisting essentially of a finely divided inorganic filler and a water-insolubilized hydrophilic organic binder sensitized with a light-sensitive benzene diazonium salt, and to a process for producing planographic printing forms by development of the coatings of such plates, after imagewise exposure thereof, so as to convert them into oleophilic azodyestuff images.

In planography orlithography as applied for offsetprinting, printing forms are used in which an oleophilic (ink receptive) image is present on a hydrophilic background. The oleophilic image is developed, for instance, on a printing plate made by applying and sensitizing a planographic coating on. a support such as an anodically oxidized aluminum foil or suitable paper. In printing, the printing form is wetted with water and ink, the water being selectively taken up by the hydrophilic areas and the ink by the oleophilic areas of the printing surface. If, however, the lithographic surface presents irregularities causing defects to occur in the water film, which must constantly be present on the hydrophilic areas, the plate starts toning." That is, the hydrophilic areas will take up ink to some extent, either at spots where the water film is interrupted or even over their whole surface, and the ink so taken up objectionably blemishes corresponding areas of the prints obtained.

In its simplest form the planographic paper plate consists of a paper support and a single coating thereon which contains a finely divided inorganic pigment and a water-insolubilized hydrophilic organic binding agent. Such single-coated plates are suitable for making a few hundred prints at most When it is desired to obtain larger runs with a planographic paper plate, one or more water repellent coatings and/or anchoring coatings are usually applied between the planographic coating and the support.

Many suggestions concerning the composition of the planographic coatings are found in literature.

Among the substances mentioned for use as the pigment in such coatings are, for instance: zinc oxide, titanium dioxide, barium sulfate, antimony trioxide, carbon black, calcium carbonate, magnesium carbonate, lithopone, aluminum oxide, calcium fluoride, magnesium oxide, calcium oxide, silica, kaolin, bentonite, aluminum silicate, diatomaceous earth, satin white, talcum, fullers earth, Georgia clay or calcined clay.

Mentioned in literature as binding agents for such coatings are, among others: gelatin, casein, 'zein, soyaprotein albumin, animal glue, hydroxyethyl cellulose, sulfoethyl cellulose, carboxymethyl cellulose cellulose sulfate, methyl cellulose, oxidized cellulose, carboxymethyl hydroxyethyl cellulose, methyl ethyl cellulose starch, hydroxyethyl starch, glucosan-2- mono-carbamate, oxidized starch, starch-acetate, enzymeconverted starch, acid-modified starch, chlorinated starch, amioca starch, starch glycolate, reaction products of hydroxyethyl starch with aminoformaldehyde resins, pectin derivatives, alginic acids, gum arabic, carragenan, chitosan, dextrin, dextrane, locust bean gum, tragacanth gum, mesquite gum, karaya gum, guar gum, polyvinyl alcohol, polyacrylic acid, polyacryl amides, methylolpolyacrylamides, copolymers of acrylic acid, methacrylic acid and their amides with hydrophobic monomers such as styrene, vinyl acetate or ethyl acrylate, polyamides, and copolymers of styrene with carboxylic acids.

For in solubilizing the binding agents a selection can be made from a great number of hardeners and/or precipitants.

.T he oleophilic image in the planographic coating can be formed either mechanically (for instance by typewriting) or photographically. On so-called presensitized printing plates, a diazo resin or an o-naphthoquinone diazide is generally used as a light-sensitive substance.

By diazo resins are meant condensation products of an aromatic diazo compound such as p-diazodiphenyl amine with an aldehyde such as formaldehyde. Upon exposure the diazo resin yields oleophilic decomposition products, and the printing form then is developed by washing away the non-decomposed diazo compound with water. Thus, printing plates sensitized'with diazo resins are normally negative working The o-naphthoquinone-diazides used in practice are mostly either water-insoluble esters that dissolve readily in organic solvents, or amides of o-naphthoquinone-diazide-sulfonic or carboxylic acids. Upon exposure they decompose and form a benzocyclopentadiene carboxylic acid which is soluble in alkaline aqueous liquids. Depending upon the composition of the light-sensitive coating and the developing method chosen, printing plates sensitized with o-naphthoquinone-diazides are either positive ornegative working. The development of them can be effected by dissolving selectively either the nondecomposed o-naphthoquinone diazide or its photmdecomposition product. With the aidof the o-naphthoquinone diazides, positive working planographic plates of high quality can be made. These plates, however, are relatively expensive, even when their support is paper, because the light-sensitive substance must be applied with the aid of an organic solvent and, consequently, the planographic coating has to fulfill special requirements in order to prevent the sensitizing liquid from penetrating too deeply. Moreover, the required developing method presents difficulties because a strongly alkaline and consequently corrosive or hazardous liquid is needed for the purpose.

There is need for inexpensive, light-sensitive, positive working printing plates enabling the production in a very simple way of planographic, or offset, printing forms suitable for making good prints through at least 50-200 printing runs. Up to now no such plate has been commercially available.

German Pat. No. 857 888 describes a process in which an anodically oxidized aluminum plate sensitized with a benzene diazonium salt, such as the chlorozincate of 4-diazonium-N- methylaniline or of 4-diazonium-2-methyl-N-ethyl aniline, is imagewise exposed and subsequently developed, according to a diazotype method, so as to convert the non-decomposed diazonium salt into an oleophilic azo-dyestuff. If the light-sew sitive coating contains an azo-coupling component, the azodyestuff is formed by exposing the plate to ammonia vapor. However, the printing form can also be developed with a solution containing an azo-coupling component which preferably is a polyvalent aromatic hydroxyl compound such as 2, 4, 2, 4-tetrahydroxyldiphenyl. On the other hand, an aluminum printing form thus obtained be development with liquid shows a blurred image, owing to bleeding of the azo-dyestuff during development. Also, the selective receptivity to ink and the adherence of the image to the plate are insufficient.

British Pat. No. l 064 128 describes a process making use of a so-called photographic baryta paper, the coating of which contains barium sulfate as a pigment and a protein binding agent and is sensitized with a light-sensitive diazonium salt of high coupling activity. After imagewise exposure, development is effected with an alkaline aqueous phloroglucinol solution and the image-bearing surface is subsequently treated with an acid solution of a hydrophilic organic binding agent, in order to render the background areas of the image sufficiently hydrophilic. This plate has too porous asurface, and the alkaline phloroglucinol solution has poor keeping quality.

British Pat. Nos. 1 038 279 and l 082 889 describe processes for making printing forms, in which a paper planographic plate sensitized for direct imaging with a benzene diazonium salt of high coupling activity is imagewise exposed and subsequently developed by means of an aqueous buffered neutral or slightly acid phloroglucinol solution. Fairly good printing forms can be obtained from the plates there disclosed. These plates, however, have a disadvantage in that the planographic coating, which contains barium sulfate as a pigment and a water-insolubilized protein as a binding agent, is too porous, as a result of which the diazo compound locally penetrates too deeply and, consequently, it is difficult to effect complete bleaching out and coupling of the diazo compound.

Upon development, the part of the diazo compound that penetrated too deeply may still form an oleophilic azodyestuff, but since this azo-dyestuff is not present on the surface it does not influence the oleophilic properties of the surface. Thus, spots which are more or less hydrophilic appear in the oleophilic image areas and manifest themselves as white dots in the image of the prints. Especially on aged plates the too deep penetration that occurs has highly undesirable effects.

It is the object of the present invention to fulfill the need for an inexpensive, light-sensitive, positive-working printing plate having a paper support, which can be converted into a desirable, durable planographic printing form in a very simple way.

According to this invention, a planographic printing plate fulfilling that need is provided by forming on a suitable paper support a planographic coating consisting essentially of an inorganic filler, or pigment, and a water-insolubilized organic hydrophilic binding agent that is principally a water-'insolubilized polyvinyl alcohol having a degree of hydrolysis greater than 98 percent, and sensitizing this coating with a light-sensitive benzene diazonium salt. The plate so produced, by being imagewise exposed and subsequently developed with an aqueous buffered phloroglucinol solution having a pH between 4 and 8, is converted into a durable, positive-working planographic, or offset, printing form that will give a large number of desirable clear prints in use.

Surprisingly, it has been found that the use of the water-insolubilized polyvinyl alcohol in the planographic coating makes it possible to obtain a valuable single-coated paperbased printing plate sensitized with a benzene diazonium salt. The single planographic layer coated onto the support adheres sufficiently to the paper surface without need for an intermediate adhesive coating; it can be sensitized without too deep a penetration of the diazo compound; and it has sufficient affinity for the azo-dyestuff to enable a run of at least 50-200 prints from the printing form produced from the plate.

As the tiller or pigment in the planographic coating,finely divided inorganic substances such as titanium dioxide, diatomaceous earth and Lustra" clay can be used, either individually or in mixtures with one another or with other pigments. The pigment is ordinarily to be made into an aqueous dispersion which in turn is mixed with a solution of the binding agent to obtain a dispersion to be coated onto the paper support. it is often advantageous to add water glass, a silica so], an alumina sol or a stannic anhydride so], or a mixture of two or more of these, to the pigment dispersion.

The polyvinyl alcohol to be used as a binding agent for the coating has a degree of hydrolysis greater than 98 percent. The binder can be rendered water-insoluble by simply drying the coating containing it in the presence of a cross-linking agent selected from among those known to be suitable for making the polyvinyl alcohol water-insoluble. Good results can be obtained with glyoxal, for instance.

The planographic coating should be hardened in such a way that it retains only a small water-absorbing capacity. Its waterabsorbing capacity can be determined satisfactorily by use of the so-called Cobb test (See: Tappi Standards T 441 os-69, or British Standard PBMA Test Method PTlS). It has been determined that the value of its water-absorbing capacity as measured according to the Cobb test should amount to less than 6. For most of the known direct imaging plates the corresponding value is considerably higher, amounting to about -20.

Since the liquid used for sensitizing the planographic coating made according to the invention does not penetrate deeply into the coating, the diazo compound is present almost exclusively on the surface of the plate. Moreover, sensitizing is uniform, i.e., spotwise penetration" of the sensitizing liquid is not apparent. in both these respects the planographic coating according to the invention differs distinctively from the planographic coatings made with another binding agent such as oxidized starch, hydroxyethyl cullulose, carboxymethyl cellulose, hydroxyethy carboxymethyl cellulose or alginic acid.

The planographic coating according to the invention preferably is applied so as to have a weight of 9-12 g./m. It is ordinarily advantageous to make it with the filler and the binding agent present in a weight ratio of between 4 and 6 parts of filler to 1 part of binding agent. The coating can be applied directly onto any paper having a sufficiently smooth surface, such as super-calendered paper.

Sensitizing of the planographic coating is preferably effected with an aqueous solution of a light-sensitive benzene diazonium salt. Since the developer to be used is a phloroglucinol solution have a pH between 4 and 8, the diazonium salt must have a relatively high coupling activity. Diazo compounds having a higher coupling activity in an aqueous solution than p-diazo o-chloro-N,N-diethyl aniline are suitable. The coupling activity can be measured in the way described in British Pat. No. 957 836. British Pat. Nos. l 064 128, l 064 129 and 347 430 and the Dutch Pat. Applications Nos. 6508566, 6708503 and 6610037 describe light-sensitive benzene diazonium salts which have a suitably high coupling activity and which can also be used with good results in sensitizing the planographic coating on account of their other properties.

Auxiliary agents can be used in the sensitizing liquid in order to improve the properties of the light-sensitive coating. The acids known to serve as stabilizers for the light-sensitive coating, such as phosphoric acid, boric acid and tartaric acid, are particularly useful. An unexpectedly good stabilization is obtained with a mixture of boric acid and tartaric acid, in which the boric acid predominates.

Development of the imagewise exposed planographic plate into a printing form is effected with an aqueous buffered solution of phloroglucinol having a pH between 4 and 8. The buffering of this solution can be effected by the use of buffer salts known to be useful in diazotype developers. These generally are alkaline-reacting salts of weak acids. For instance, good results can be obtained with alkali metal, ammonium or alkaline earth metal tartrates, adipates, acetates, phosphates and citrates as buffers. [t is also advantageous to provide in the developing solution 1-l0 percent by volume of water-miscible organic solvent having a molecular weight above 50 and preferably a boiling point about C., such as propyl glycol, acetonyl acetone, diacetone alcohol, N-methylpyrrolidone-Z, or others, according to disclosures of a copending application, Ser. No. 790,175, filed Jan. 9, l969.

The imagewise exposure is preferably effected by exposing the light-sensitive printing plate, in contact with a transparent original, to actinic light. Printing forms achieving a run of to 250 prints can be obtained even from originals obtained by typewriting on translucent paper. When the original used is produced by typewriting on a translucent sheet having good light-permeability, for instance, on a coated sheet the support of which is a thin polyester or cellulose acetate film and the coating of which consists principally of an inorganic transparent pigment such as amorphous or crystalline silica and a hydrophobic organic film-forming binding agent such as polyvinyl acetate or cellulose acetate-butyrate, some thousands of copies can be made from the printing form obtained, if the weight ratio between pigment and binding agent is greater than 0.8. This ratio is preferably between 1 and 3.

The principles and results of the invention and preferred ways of carrying it out will be further evident from the following illustrative examples.

EXAMPLE I Super-calendered paper having a weight of 106 g./m. is coated with a dispersion which contains:

500 ml. of a solution of 66 g. of polyvinyl alcohol having a degree of hydrolysis of 99.8 percent in 1,000 ml. of water,

220 ml. of an aqueous dispersion containing per liter 730 g. of titanium dioxide doped with A1 0 and Si0 and having an average particle size of about 0.2g,

1 ml. of water glass,

81 ml. of a 40 percent solution of glyoxal in water,

55 ml. of isopropanol and water in a amount bringing the volume up to 1.000 ml.

The coating is applied with sufficient thickness that it weighs 12 g./m. upon being dried and hardened by heating it for to sec. at 155 to 160 C.

Of each of the following four diazo compounds:

a. 4-benzoylamino-2,S-di-n-butoxybenzene umchloride, zinc chloride double salt,

b. 4-(4-methoxyphenylthio)-5-methoxy-2-N-methyl, N-

ethoxycarbonylamino benzene diazoniumchloride, zinc chloride double salt,

c. 4-dimethylamino-3-chloro-2(4" benzene umchloride, zinc chloride double salt, and

d. 4-bis- (2-acetoxyethyl)amino-3-(2, 4, 5'-trichloro phenoxy)-6-chlorobenzene diazoniumchloride, zinc chloride double salt,

0.06 mol is dissolved in a liquid which contains 8.3 g. of boric acid and 1.7 g. of tartaric acid dissolved in 400 ml. of isopropanol and 600 ml. of water.

A plate prepared as above described in this Example 1 is sensitized with each of the sensitizing liquids, so that each plate contains about 0.5 millimol of diazo compound per m after drying. The sensitized plates are marked a, b, c and d, respectively.

The light-sensitive printing plates are imagewise exposed under a translucent paper original provided with a strongly UV-absorbing typewritten text, until all the diazo compound has bleached out under the imagefree areas of the original. Subsequently each plate is developed with a solution which contains:

8 g. of phloroglucinol,

4 g. of thiourea,

2 g. ofcitric acid,

50 g. of trisodium citrate (2 aq),

I g. of potassium hydroquinone monosulfonate,

100 g. of sodium chloride,

60 ml. of propylglycol and water up to 1000 ml.

The pH of this solution is about 6.5.

Immediately after the development the plates are clamped on an offset printer and run thereon to make prints on paper sheets. Plate a has a run of 150, plate b of 200, plate c of 100 and plate d of 250 good prints.

diazonidiazoni- EXAMPLE II The following dispersions are prepared:

a. A dispersion which contains:

600 ml. of a solution containing 100 g. of polyvinyl alcohol having a degree of hydrolysis of 99-100 percent, in 1,000 ml. of water,

250 ml. of an aqueous dispersion containing 730 g./l of titanium dioxide doped with A1 0 and SK); and having an average particle size of about 0.2g,

90 ml. of glyoxal solution (45 percent),

50 ml. of isopropanol and 60 ml. of an alkali-stabilized aqueous dispersion of silica having an average particle size between 7 and 30 nm.

b. A dispersion as described under a) in which, however, 600 m. of a solution containing 100 g. of oxidized starch in 1,000 ml. of water has been used instead of the 600 ml. of polyvinyl alcohol solution.

c. A dispersion as described under a) in which, however, the 600 ml. of polyvinyl alcohol solution has been replaced by 600 ml of a solution which contains 30 g. of the sodium salt of carboxymethyl hydroxyethyl cellulose (carrying 0.3 carboxymethyland 0.7 hydroxyethylgroups per glucose unit) in 1,000 ml. of water.

. A dispersion as described under a) in which, however, the

600 ml. of polyvinyl alcohol solution has been replaced by 600 ml. of a solution containing 100 g. of sodium carboxy-methyl cellulose in 1,000 ml. of water.

A dispersion as described under a) in which, however, the

600 ml. of polyvinyl alcohol solution has been replaced by 600 ml. of a solution containing 66 g. of hydroxyethyl cellulose in 1,000 ml. of water.

f. A dispersion as described under a) in which, however, the 600 ml. of polyvinyl alcohol solution has been replaced by 600 ml. of a solution containing 50 g. of sodium alginate in 1,000 ml. of water.

With each of the six dispersions supercalendered paper having a weight of 106 g./m. is treated so that, after drying and hardening the coating, a coating weighing about 10 g./m. is present on each sheet. The plates are marked a, b, c, d, e and f, respectively. The six plates thus obtained are each sensitized with a solution which contains:

30 g. of 4-(4'-methoxyphenylthio)-5-methoxy Z-N-methyl, N-ethoxycarbonyl amino benzene diazoniumchloride, zinc chloride double salt,

8 g. of boric acid,

1.6 g. of tartaric acid, and

300 ml. of isopropanol in 700 m. of water; and each plate is then dried.

The plates are imagewise exposed and subsequently developed with a solution containing:

12 g. of phloroglucinol,

15.3 g. of citric acid,

87 g. of disodium phosphate (2 aq),

50 g. of sodium chloride,

50 ml. of n-propanol and water up to 1,000 ml.

Printing is effected by an offset printer from the six printing forms obtained. Only the printing form of plate a will give a good result.

EXAMPLE III a. the following dispersion is prepared:

250 ml. of an aqueous dispersion containing 730 g./l of titanium dioxide which has not been doped and has an average particle size of about 0.214.,

600 ml. of an aqueous solution containing 100 g./l of polyvinyl alcohol having a degree of hydrolysis of 99.8 percent,

90 ml. of glyoxal solution (40 percent), 50 ml. of

isopropanol and water up to 1,000 ml.

. The following dispersion is prepared:

250 ml. of an aqueous dispersion containing 650 g./l of Lustra" clay (average particle size 0.5-2p.),

600 ml. of an aqueous solution containing g. per liter of polyvinyl alcohol having a degree of hydrolysis of 99.8 percent,

90 ml. of glyoxal solution (40 percent),

50 ml. of isopropanol and water up to 1,000 ml.

c. The following dispersion is prepared:

250 ml. of an aqueous dispersion containing 400 g./l of diatomaceous earth (average particle size l-3 u),

600 ml. of an aqueous solution containing 100 g./l of polyvinyl alcohol having a degree of hydrolysis of 99.8 percent,

90 ml. of glyoxal solution (40 percent),

50 ml. of isopropanol and water up to 1,000 ml.

With each of the dispersions a), b), and c) supercalendered paper is provided with a planographic coating weighing about 9 g./m The plates are sensitized as described in Example II. After drying, an oleophilic azo-dyestuff image is formed on each of the light-sensitive plates by imagewise exposure and development. Development is effected with a solution containing:

12 g. of phloroglucinol,

l l g. of citric acid, and

87 g. of disodium phosphate (2 aq) in 1,000 ml. of water.

Each of the three printing forms thus produced givesa run of about 200 prints.

It is to be understood that the invention herein disclosed and defined by the appended claims may be carried out in ways other than those particularly mentioned above or illustrated by the examples. The invention is not intended to be limited by particulars of the foregoing description or examples except as may be required for fair interpretation of the claims.

What is claimed is:

l. A sensitized plate for conversion into a planographic printing form, comprising a paper support carrying thereon a planographic coating consisting essentially of a layer of a water-insolubilized hydrophilic organic binder having a finely divided inorganic filler dispersed therein and a light-sensitive diazo compound applied to said layer, wherein said binder consists principally of a substantially water-insoluble product of the cross-linking, in situ in said layer to such extent that the water-absorbing capacity of said layer is less than about 6 as measured by the Cobb test, of polyvinyl alcohol having a degree of hydrolysis greater than 98 percent in admixture with a cross-linking agent for said polyvinyl alcohol and said diazo compound is a non-polymeric benzene diazonium salt that has a higher coupling activity in aqueous solution than p-diazo-ochloro-N,N-dimethyl aniline and that forms an ink-receptive azo dyestuff by coupling with phloroglucinol when said coating is developed with an aqueous buffered phloroglucinol solution having a pH between 4 and 8.

2. A sensitized plate according to claim 1, said layer containing, by weight, 4-6 parts of said filler to 1 part of said binder.

3. A sensitized plate according to claim 1, said layer being adhered by said binder directly to the paper surface of said support.

4. A sensitized plate according to claim 1, said layer having said diazonium salt applied thereto in admixture with an acidic stabilizer comprising a mixture of boric acid and tartric acid in which the boric acid predominates.

5. A sensitized plate according to claim 1, said cross-linking agent comprising glyoxal provided initially in said layer in admixture with said polyvinyl alcohol.

6. A sensitized plate according to claim 1, said salt being a benzoylamino-dibutoxy-benzene diazonium salt, a phenylthiobenzene diazonium salt or a bis-(acetoxy-ethyl) aminochlorophenoxy-chloro-benzene diazonium salt.

7. A sensitized plate according to claim 1, said salt being a dimethylamino-chloro-(4' methyl-phenyl)thio-benzene diazonium salt.

8. A sensitized plate according to claim 1, said filler consist ing principally of pigmentary titanium dioxide, diatomaceous earth or clay or a mixture thereof.

9. A sensitized plate according to claim 1, said filler consisting principally of pigmentary titanium dioxide diatomaceous earth or clay or a mixture thereof in admixture with a minor proportion of alumina, silica, water glass or stannic anhydride or a mixture thereof.

10. A sensitized plate for conversion into a planographic printing form, comprising a paper support carrying thereon a planographic coating consisting essentially of a layer of a water-insolubilized hydrophilic organic binder having a finely divided inorganic filler dispersed therein and a light-sensitive diazo compound applied to said layer in admixture with an acidic stabilizer, said layer containing, by weight, about 4-6 parts of said filler to 1 part of said binder and being adhered by said binder directly to the paper surface of said support, said binder consisting principally of a substantially water-insoluble product of the cross-linking, in situ in said layer, of polyvinyl alcohol having a degree of hydrolysis greater than 98 percent in admixture with glyoxal as a cross-linking agent other than said polyvinyl alcohol itself, said diazo compound being a phenylthio-benzene diazonium salt that has a higher coupling activity in aqueous solution than p-diazo-o-chloro-N,N- dimethyl aniline and that forms an ink-receptive azo dyestuff by coupling with phloroglucinol when said coating is developed with an aqueous buffered phloroglucinol solution having a pH between 4 and 8.

11. A sensitized plate according to claim 10, said polyvinyl alcohol having been cross-linked to such extent that the waterabsorbing capacity of said layer is less than about 6 as measured by the Cobb test.

12. A sensitized plate according to claim 10, said diazo compound being a dimethylamino-chloro-(4' methylphenyl)thio-benzene diazonium salt.

13. A sensitized plate according to claim 10, said filler consisting of principally of pigmentary titanium dioxide, diatomaceous earth or clay or a mixture thereof in admixture with a minor proportion of alumina, silica, water glass or stannic anhydride or a mixture thereof.

14. A planographic printing form comprising a paper support carrying thereon a planographic coating consisting essentially of a layer of a water-insolubilized hydrophilic organic binder having a finely divided inorganic filler dispersed therein and an image of an oleophilic azo-dyestuff formed on said layer, wherein said binder consists principally of a substantially water-insoluble product of the cross-linking, in situ in said layer, of polyvinyl alcohol having a degree of hydrolysis greater than 98 percent in admixture with glyoxal as a crosslinking agent and said dyestuff is a reaction product of a nonpolymeric benzene diazonium salt applied to said layer, said salt having a higher coupling activity in aqueous solution than p-diazo-o-chloro-N,N-dimethyl aniline, and phloroglucinol.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION ?atent No. 3,658,538 Dated April 25, 1972 Aloysius Henricus Jacobus Hilhorst Assignor to Van Der Grinten N.V.

' of venlo, Netherlands It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 2, line A6, "be" should read by Col. 5, line 2, "a" should read an Col. 5, line 12. M should read -methy1phen'y1) thio Col. 5, line 61, m" should read m1 Col. 6, line 19, "m" should read ml Claim r, line 3., "tartric" should read tartaric Signed and sealed this 27th day of March i973.

(SEAL) Attest:

"1 Attesting Officer I M.FLETCHER,JR. ROBERT GOTTSCHALK Coumissioner of Patents 

2. A sensitized plate according to claim 1, said layer containing, by weight, 4-6 parts of said filler to 1 part of said binder.
 3. A sensitized plate according to claim 1, said layer being adhered by said binder directly to the paper surface of said support.
 4. A sensitized plate according to claim 1, said layer having said diazonium salt applied thereto in admixture with an acidic stabilizer comprising a mixture of boric acid and tartric acid in which the boric acid predominates.
 5. A sensitized plate according to claim 1, said cross-linking agent comprising glyoxal provided initially in said layer in admixture with said polyvinyl alcohol.
 6. A sensitized plate according to claim 1, said salt being a benzoylamino-dibutoxy-benzene diazonium salt, a phenylthio-benzene diazonium salt or a bis-(acetoxy-ethyl) amino-chlorophenoxy-chloro-benzene diazonium salt.
 7. A sensitized plate according to claim 1, said salt being a dimethylamino-chloro-(4'' methyl-phenyl)thio-benzene diazonium salt.
 8. A sensitized plate according to claim 1, said filler consisting principally of pigmentary titanium dioxide, diatomaceous earth or clay or a mixture thereof.
 9. A sensitized plate according to claim 1, said filler consisting principally of pigmentary titanium dioxide diatomaceous earth or clay or a mixture thereof in admixture with a minor proportion of alumina, silica, water glass or stannic anhydride or a mixture thereof.
 10. A sensitized plate for conversion into a planographic printing form, comprising a paper support carrying thereon a planographic coating consisting essentially of a layer of a water-insolubilized hydrophilic organic binder having a finely divided inorganic filler dispersed therein and a light-sensitive diazo compound applied to said layer in admixture with an acidic stabilizer, said layer containing, by weight, about 4-6 parts of said filler to 1 part of said binder and being adhered by said binder directly to the paper surface of said support, said binder consisting principally of a substantially water-insoluble product of the cross-linking, in situ in said layer, of polyvinyl alcohol having a degree of hydrolysis greater than 98 percent in admixture with glyoxal as a cross-linking agent other than said polyvinyl alcohol itself, said diazo compound being a phenylthio-benzene diazonium salt that has a higher coupling activity in aqueous solution than p-diazo-o-chloro-N,N-dimethyl aniline and that forms an ink-receptive azo dyestuff by coupling with phloroglucinol when said coating is developed with an aqueous buffered phloroglucinol solution having a pH between 4 and
 8. 11. A sensitized plate according to claim 10, said polyvinyl alcohol having been cross-linked to such extent that the water-absorbing capacity of said layer is less than about 6 as measured by the Cobb test.
 12. A sensitized plate according to claim 10, said diazo compound being a dimethylamino-chloro-(4'' methylphenyl)thio-benzene diazonium salt.
 13. A sensitized plate according to claim 10, said filler consisting of principally of pigmentary titanium dioxide, diatomaceous earth or clay or a mixture thereof in admixture with a minor proportion of alumina, silica, water glass or stannic anhydride or a mixture thereof.
 14. A planographic printing form comprising a paper support carrying thereon a planographic coating consisting essentially of a layer of a water-insolubilized hydrophilic organic binder having a finely divided inorganic filler dispersed therein and an image of an oleophilic azo-dyestuff formed on said layer, wherein said binder consists principally of a substantially water-insoluble product of the cross-linking, in situ in said layer, of polyvinyl alcohol having a degree of hydrolysis greater than 98 percent in admixture with glyoxal as a cross-linking agent and said dyestuff is a reactiOn product of a non-polymeric benzene diazonium salt applied to said layer, said salt having a higher coupling activity in aqueous solution than p-diazo-o-chloro-N,N-dimethyl aniline, and phloroglucinol. 